Disk drives for magnetic disk storage devices require an exceedingly high standard of cleanliness. The sealing of the disk drive chamber is essential to maintain the extremely high standard of cleanliness within the disk chamber.
Contaminants, which typically tend to migrate into the disk drive chamber, include dust particles and the residue and vapors from the spindle motor and bearings of the disk drive storage device. Further, the lubrication media, oils or greases, used in the bearings of the motor tends to give off aerosols or vaporous components of the lubricants; without an adequate seal, these aerosols and vapors would tend to migrate into the disk drive chamber.
To prevent the migration or these contaminants into the disk chamber of the disk drive storage device, it is essential that a reliable seal be placed between the motor chamber and the disk chamber. Magnetic seals have been widely used in this environment, wherein an annularly shaped magnetic arrangement is placed in an annular space around the shaft to be sealed and the gap between the pole pieces of the annular magnetic arrangement and the shaft are closed with a ferromagnetic fluid. Typically, the seal rotates about a fixed shaft. The fixed shaft is a portion of the motor which drives the disk drive hub. The magnetic storage disks typically are attached by conventional means to the hub and rotated therewith. However, the seal may be implemented in a rotating shaft drive design.
As the seal assembly rotates, the movement of the seal assembly and, particularly, the pole pieces around the shaft will cause movement of the ferromagnetic fluid through the viscous action of the fluid adhering to the pole piece and to the shaft and being revolved with the rotating part. With this movement of at least some of the ferromagnetic material around the shaft, centrifugal forces act to displace quantities of this fluid outwardly from the shaft and thus displace the fluid from the seal gap.
Once the fluid has been displaced from the seal gap, the magnetic influence of the magnetic seal structure on the ferromagnetic fluid is lost or at least overcome; the fluid is then free to respond to further forces and either to migrate into the motor cavity and potentially foul the motor bearing lubricant, or to move into the disk cavity or chamber and contaminate the magnetic storage disks and other internal components.
U.S. Pat. No. 4,701,653 to Merkle, et al; U.S. Pat. No. 4,779,165 to Elsaesser, et al; U.S. Pat. No. 4,817,964 to Black, Jr. and U.S. Pat. No. 4,907,897 to Shirotori all disclose magnetic seals for magnetic disk storage units but fail to disclose any shields associated with the magnetic seal which would assist in any way with the ferromagnetic fluid containment in the seal gap.
An attempt to prevent or suppress the migration of ferromagnetic seal fluid has been made in a magnetic seal design by Nidec Corporation, where a single shield was associated with a magnet and two magnetic pole pieces in an attempt to reduce magnetic fluid migration into the disk chamber of the magnetic storage disk device. The design of the prior art seal is illustrated in FIG. 1.
One of the shortcomings of the design shown in FIG. 1 is that the interface between the shield and the adjacent magnetic pole piece acts as a capillary which captures, holds and transports any magnetic fluid which is removed by centrifugal force from the seal gap into the capillary, and thus retains the ferromagnetic fluid in a region from which the ferromagnetic fluid may never be recovered to be repositioned in the seal gap.
Further, the prior art design of FIG. 1 lacks any ability to maintain any magnetic fluid displaced by centrifugal forces from the bottom pole of the seal. Accordingly, that displaced fluid inherently will be dispersed into the motor cavity of the device, lending the possibility that contamination of the motor assembly and subsequent malfunction of the motor or bearings can occur.
Additionally, the assembly integrity of the prior art design illustrated in FIG. 1 relies upon an adhesive seal between the bottom pole and the shield plus a second adhesive seal between the shield and the hub.
All the elements, including both top and bottom pole pieces, the magnet and the shield, have slip fits with respect to the surfaces within which they are positioned. With a slip fit, the magnet assembly must be sealed into the shield by some type of an adhesive seal material and the shield then must be attached to the hub by an adhesive seal material. Any defects in the adhesive seals of the pole pieces, shield and magnet may result in leakage of the magnetic fluid. Additionally, the prior art design of FIG. 1 has no seal of any material between the top pole and the top shield.
U.S. Pat. No. 5,009,436 to Endo, et al, and assigned to Nippon Denson Corp., illustrates a shield arrangement on one side of a magnetic seal using a magnetic shield to create a secondary control over the flux density near the pole pieces of the magnetic seal. This secondary magnetic flux path to the shaft may attract and hold ferromagnetic fluid in a secondary region which will, at the same time, deplete or reduce the sealing fluid supply at the primary seal gap. Magnetic repulsion is relied upon to prevent the fluid from escaping the seal.